The present invention relates to a fastening system for fastening a member to a structural metal part and, more particularly, for fastening a member to sheet metal. A threaded metal stud having an annular welding surface is fastened to the structural part in short-time arc welding, and a fracturable lock nut is screwed onto the stud to fasten the member to the structural part.
Metallic weld stud are typically solid, non-compressible bodies which are formed by standard fastener stamping methods such as being cold headed from rod materials. Such weld studs are welded to a component surface by using a known welding device which provides energy through the weld stud to melt both a circular sacrificial weldment element, as well as a portion of the component base material. The weld stud is fixed to the base material due to the commingling and cooling of the liquefied metals. The welding device, in particular a weld gun, grasps the weld stud using clamping jaws in a region between a shoulder of the stud and the end of the face to be welded. Each weld stud is placed into the mouth of weld gun one at a time immediately prior to welding. The size, weight, and configuration of these studs prevent their use in blow fed weld guns.
Increased fuel economy demands within transportation industries have lead to the use of thinner and thinner gauge materials. In particular, metallic composite materials and particularly aluminum composite materials with a polymer core have effectively been used as surface or skin materials for various structures. These very thin metallic laminates are typically coupled or fastened to a support structure by being fastened to the support structure on one side of the laminate. To this end, it is known to fasten a ordinary weld stud to a the laminate structure to facilitate the coupling of the stud to the structure.
Traditionally, weld studs burn through at least the first layer of the thin laminate material. The weldment provides a stress riser which significantly weakens the strength of weld stud laminate interface. Repeated loading of the weld stud leads to plastic deformation in the coupling interface and premature failure of the stud to laminate interface.
Known studs have a head and shank connected to the head. The head of a stud is welded to the sheet metal of a vehicle body. The welding operation takes place according to the known method of drawn-arc welding. In this method, the stud is brought into contact with the sheet metal, the welding current is then switched on and the stud is drawn off the sheet metal so that an arc is formed between the stud and the sheet metal. While the arc is burning, part of the stud head and part of the sheet metal melt. When a sufficient amount of molten metal has been generated, the stud is driven into the melting. The welding operation as such can be influenced by several parameters. The influence of the individual parameters has various effects on susceptibility to error when welding studs with the drawn-arc method.
For the purpose of arc initiation, the head geometry of the stud must be designed accordingly. In this context, studs having a head with a conical tip are known in the art. Studs with a substantially flat head/front end are further know, with an ignition tip formed in the center of the front end. Studs with a flat front end are further known in the art. The threaded stud is welded onto a metal sheet of the body in so-called short-time arc welding. Short-time arc welding is also known as stud welding, where a metal stud (threaded stud) is positioned so as to contact the sheet metal of the body. A pilot current is then turned on and the metal stud is again slightly lifted off from the sheet metal of the body. At the same time, an arc is drawn. Then, a welding current is turned on so that the facing surfaces of metal stud and body sheet metal are fused. The metal stud is then again lowered onto the sheet metal of the body so that the melts combine. The welding current is turned off and the whole fused mass solidifies.
A lock nut is then typically screwed onto the stud, thus projecting from the sheet metal of the body. The nut acts to fix the member to the sheet metal. As a rule, the lock nut is made of synthetic material. The stud may be a coarse-pitch threaded stud or a fine-pitch threaded stud. A matching thread is provided on the lock nut. In the case of a coarse-pitch thread, it is alternatively possible that only one hole is provided on the lock nut. The coarse-pitch thread then cuts a corresponding counter-thread into the hole.
Fractures of threaded studs and of metal body sheet occur in undefined fashion. It is hard to establish what the reason for the failure was. In addition, reworking of the fractured sheet metal of a car body requires a considerably greater expenditure than reworking in the case of a fractured stud. In a fracture of the stud, a new stud can be welded at the same spot, without the strength of the sheet metal suffering.
Against this background, the problem underlying the invention is to provide an improved fastening system of generic type which, in particular, requires little reworking in the situation when the coupling of a nut to the welded stud structure fails.
To overcome the disadvantages of the fastening system mentioned, the strength of the welded joint between the structural part and the threaded stud and the strength of the stud and nut themselves are adapted to one another. Specifically, they are adapted so that upon application of a torque that exceeds that torque, which is applied per specification when the lock nut is screwed onto the threaded stud, it is ensured that the nut fractures before the stud fractures and in circumstances where the nut does not fracture, the stud fractures before the structural part fails.
This ensures that whenever too high a torque is applied to a threaded stud having a xe2x80x9cgoodxe2x80x9d welded joint, the nut or the stud, and not the structural part, fractures in every case. In this way, reworking costs due to incorrectly adjusted torque or tension wrenches or threading problems with the fastener are reduced. Even when a lock nut having too high a strength is used, it is ensured that damage of the structural part is largely ruled out when the welded joint between the stud and the part is xe2x80x9cgood.xe2x80x9d
In this regard, a xe2x80x9cfracturexe2x80x9d is intended to mean any damage to an element (lock nut, stud, structural part) in which a torque applied to the respective element can no longer be transmitted to a following element of the fastening chain. A fracture of the structural part generally is intended to signify that the part is structurally damaged and, in particular, that it pulls out in the region of the welded joint. In this way, the object is fully accomplished.
This embodiment has the advantage that strengthening of the structural part (sheet metal of the body of the vehicle) is unnecessary to ensure that, upon application of an excessively high torque, the stud will fracture before the part fractures.
According to another preferred embodiment, the threaded stud has a flange section that is arranged in the neighborhood of the welded joint and against which the member is screwed by the lock nut or against which the lock nut itself is screwed.
According to an additional preferred embodiment, the stud is a coarse-pitch threaded stud whose external thread, when the lock nut is screwed on, cuts a thread into its hole. According to an alternative embodiment, the threaded stud has a fine-pitch thread such as a metric thread and the lock nut has a corresponding internal thread.
In addition, it is preferable when the strength of the threaded stud and the strength of the lock nut are adapted to one another in such a way that, upon application of a torque to the lock nut that exceeds that torque which per specification is applied when the lock nut is screwed onto the threaded stud, it is ensured that the lock nut fractures before the stud fractures.
On the whole, in this way a closed process chain is obtained in which the predetermined breaking moment of the lock nut is smaller than the predetermined breaking moment of the threaded stud, which in turn is smaller than the predetermined breaking moment of the structural part and/or of the welded joint between the structural part and the stud.
In accordance with the teachings of another embodiment, there is provided a weld stud assembly for use with a drawn arc welding system that overcomes the deficiencies of the prior art. The weld stud assembly has head having an annular weldment area. The annular weldment area has an exterior radius which conforms to the exterior radius of the head. The annular weldment area functions to provide a weldment surface area which is about equal to the surface area from the typical circular weldment are while improving the distribution of torsional stud loads into the sheet metal.
The features mentioned above and to be explained below are usable not only in the combination indicated in each instance, but are also usable in other combinations or standing alone, without exceeding the scope of the present invention. In accordance with the teachings of the present invention, there is provided a weld stud assembly for use with a drawn arc welding system that overcomes the deficiencies of the prior art. The weld stud assembly has head having an annular weldment area. The annular weldment area has an exterior radius which conforms to the exterior radius of the head. The annular weldment area functions to provide a weldment surface area which is about equal to the surface area from the typical circular weldment are while improving the distribution of torsional stud loads into the sheet metal.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.